*{
    margin: 0;
    padding: 0;
}

body, html{
    height: 100%;
}
.box{
    display:flex;
    flex-direction: row;
    justify-content: flex-start;
}
.left{
    background-color:yellow;
    width: 30%;
    height: 50rem;
}
.right{
    background-color: blue;
    flex: 1;
}